A variety of chemical compositions are used as emulsifying agents in food, cosmetics, paint, pharmaceutical and polymer industries, as well as in textile and leather processing, ore flotation, oil drilling and agricultural spraying operations. In many of these applications, the emulsifying agent also functions as a stabilizer of the viscosity or fluidity of the continuous phase. Frequently, these applications require that the emulsion be shelf stable for long periods of time.
Typical compositions which function as water soluble emulsifiers and stabilizers include guar gum, gum arabic, and other gums, starches, proteins, various water soluble polymers, and the like. (See, Encyclopedia of Chemical Technology, Kirk-Othmer Editor, 3rd Edition, Wiley-Interscience, New York, N.Y., 1979, Vol.8, pp. 900-910, 918, 923-925; Vol. 12, pp. 55-6.) Gum arabic is preferred in many applications for its shelf stability, particularly during refrigerated or frozen storage of the emulsion.
Gum arabic is a branched, substituted heteropolysaccharide characterized by extreme water solubility, low viscosity, and the absence of odor, color or flavor. Gum arabic has been used as an emulsifier and stabilizer in foods such as confections, syrups, flavor oil emulsions, ice cream and beverages, and in inks, adhesives, textiles and lithography solutions.
Gum arabic is a naturally occurring gum which is grown in the Middle East and Africa. Because gum arabic is obtained from these areas, it is expensive, and its supply and quality are unpredictable. Accordingly, industry has long searched for a shelf stable, low cost replacement for gum arabic, and starch derived products have been suggested for such use.
U.S. Pat. No. 2,661,349 issued Dec. 1, 1953 to Caldwell et al. discloses substituted dicarboxylic acid anhydride starch half ester derivatives. Certain of these derivatives form stable oil-in-water emulsions suitable for use in beverage emulsion, flavor emulsion, and other emulsion applications. (See, e.g., P. Trubiano, Chapter 9, in Modified Starches: Properties and Uses, CRC Press, Boca Raton, Fla., 1986, p. 134-47.) Cold water soluble, low viscosity octenylsuccinate starch derivatives have been successfully used to replace gum arabic in carbonated beverages. Higher viscosity octenylsuccinate derivatives have been useful as gum arabic replacers in salad dressings. Such substituted dicarboxylic acid starch half ester derivatives also have been used in place of gum arabic to encapsulate hydrophobic substances such as flavors, vitamins, fragrances and oils. The encapsulation is typically prepared by spray-drying an oil-in-water emulsion. Certain of these encapsulating agents have been modified to yield a composition which provides a gradual or controlled release of the entrapped flavor or oil. Others can be dissolved in water at higher solids than their gum arabic counterparts and may be superior to gum arabic in certain applications.
The low viscosity (converted) starches which are used in beverage and flavor emulsions are ordinarily prepared by acid degradation of the base starch. Processes for producing low viscosity starches are well-known. U.S. Pat. No. 4,035,235 issued Jul. 12, 1977 to Richards et al. discloses a method for degradation of lipophilic substituted starches which uses alpha-amylase digestion as an alternative to acid degradation for preparing low viscosity starches. The lipophilic substituted starches are suitable for flavor encapsulation and oil-in-water emulsions. These conversion methods produce starch products which are suitable as emulsifiers and encapsulating agents for oils used in beverages.
One drawback to the use of the known starch derived products in replacing gum arabic is that known starch derivatives are less stable during storage. These starch derivatives display shorter shelf-life and poorer refrigeration and freeze/thaw stability than gum arabic. Therefore, in certain applications, such as flavored syrup bases used in the manufacture of soft drinks and similar types of beverages, these starch derived substitutes do not perform as well as gum arabic. Because beverage manufacturers ship flavored syrup bases to bottlers in diverse locations where the syrup bases may be held in refrigerated storage for long periods of time prior to use in the bottling operation, the flavor oil emulsion must remain stable during storage. In addition, because refrigeration temperatures may vary from bottler to bottler, or from day to day, the flavor oil emulsion must be able to withstand temperature cycling, including freeze/thaw cycles.
The stability problem in beverage applications is thought to occur because of the tendency of starch products to retrograde, causing the flavor oil emulsion to break down upon temperature cycling or long term storage. In severe cases, the starch may retrograde to form a gel and the flavor oil may separate entirely from the water phase. Retrogradation of starches is essentially a crystallization process that occurs when linear portions of the starch molecules align themselves next to each other and form interchain hydrogen bonds through the hydroxyl groups. When sufficient interchain bonding occurs, the molecules associate to form molecular aggregates which display a reduced capacity for hydration and, therefore, lower water solubility. These aggregates may precipitate, or, in more concentrated solutions, may form a gel. The tendency to retrograde is more pronounced in starches containing high levels of the linear amylose molecule. In starches containing both linear (amylose) and branched (amylopectin), or only branched molecules, the tendency to retrograde is less pronounced. As the temperature is lowered, both amylose and amylopectin containing starches display a greater tendency to retrograde.
Retrogradation has been partially overcome in certain applications by chemically derivatizing the starch molecule to stabilize the starch by interfering with the association between starch molecules, or portions of the same molecule, and thereby reducing the tendency of the starch to lose its hydration ability on storage. For example, reacting the starch with a reagent to introduce substituents such as hydroxypropyl, phosphate, acetate or succinate groups tends to stabilize the starch molecule during storage. These derivatization reactions may be carried out on starches which are further modified by crosslinking or degradation to obtain starches for particular applications. However, these derivatized starches do not provide the stable emulsification properties which are typical of gum arabic.
Other processes known to limit starch retrogradation at low temperatures also do not provide stable emulsifying starches. For example, U.S. Pat. No. 3,525,672 issued Aug. 25, 1970 to Wurzburg et al., discloses treating a crosslinked, inhibited starch thickener with an enzyme such as beta-amylase to impart freeze/thaw stability to starch thickeners for pie fillings, puddings and other thickened foods which are subjected to low temperature storage. It is stated that in addition to the described inhibition procedure, it is sometimes advantageous to partially derivatize the starch bases. Typical substituent groups include ester groups such as acetate, succinate, phosphate and sulfate groups as well as ether groups.
U.S. Pat. No. 4,428,972 issued Jan. 31, 1984 to Wurzburg et al., discloses a waxy starch thickener with superior low temperature stability in aqueous dispersions, which starch is obtained from a selected plant of a wxsu.sub.2 genotype.
However, none of these inventions for overcoming retrogradation during low temperature storage disclose a starch product useful for preparing a shelf stable oil-in-water emulsion. Thus, there remains a need for a product which combines the properties of emulsification with stability during shelf storage, refrigeration and freeze/thaw cycles, and which may be used to replace gum arabic.
Accordingly, it is an object of the present invention to provide an improved modified starch which exhibits the stable emulsification characteristics of gum arabic. It is a further object to provide a method for producing such a modified starch.
It is a further object to provide products containing starch based emulsions having good shelf stability, particularly foods and beverages containing flavor oil emulsions.